1. Field of the Invention
The present invention relates to an electronic part using a material with microwave absorbing properties. To be more specific; the present invention relates to an electronic part that attenuates frequency components in the high frequency range by absorption. The term electronic part, as defined in the present invention, includes, at least, a signal transmission element, a connector and a circuit board.
2. Discussion of Background
A low frequency pass type (high frequency stopping type) signal transmission element is typically a low pass filter. A typical low pass filter in the prior art obtains the required filter characteristics by taking advantage of differences in the frequency characteristics between impedance matching and impedance mismatching to reflect a signal that belongs to a frequency band in the high frequency range. Because of this, the undesired frequency component which has been reflected is sometimes returned to the front of the filter, which may cause, for instance, an unexpected oscillation within the circuit. An absorption type low pass filter absorbs the undesired frequency component and it remedies the problem described above, which is observed in a reflection type low pass filter.
There are absorption type low pass filters already in the known art. For instance, there is an absorption type low pass filter, already in wide use, that uses ferrite; to be more specific, ferrite beads. However, the frequency band that can be absorbed with ferrite is in the frequency range below 2 GHz. This means that, since the absorbing effect is not significant at approximately 2 GHz or higher, transmission of signals in this range is allowed.
Another approach is disclosed in U.S. Pat. No. 4,297,661 with a high pass filter whose microstrip is constituted with ferrite. This high pass filter takes advantage of the phenomenon that an absorbing effect is generated in the low frequency range and not in the high frequency range. However, this prior art technology, too, does not suppress the undesired signal component in the high frequency range of 1 GHz or higher by absorption.
Schiffres proposed a coaxial transmission line using ferrite in IEEE Transaction on Electromagnetic Compatibility, EMC--6, 55-61, 1964. This coaxial transmission line, however, was designed mainly for acquiring properties in the MHz band; transmission properties and reflecting properties in the high frequency range of 1 GHz or higher were not disclosed. It is assumed that signals in the high frequency range of 1 GHz or higher are transmitted.
A combination of a non-magnetic material which has absorbing qualities in the high frequency range and a ferrite for signal removal by absorption in the high frequency range has been reported as an attempt at signal removal. This approach includes the EMI filter proposed by Schlicke in IEEE Spectrum, 59-68, 1967 and the low pass type EMI filter proposed by Bogar in Proc. of IEEE 67 159-163 1979. In these filters, part of the insulator of the coaxial filter is constituted by laminating ferrite and a dielectric substance. In addition, Fiallo proposed, in his doctorate thesis at Pennsylvania State University in 1993 and in IEEE, Transactions on Microwave Theory Tech., MTT--42 1176, 1984, a filter with a microstrip structure that combines a ferrite and a dielectric substance. However, these prior art technologies require a complex multiple layer structure.
U.S. Pat. No. 4,146,854 discloses an attenuating element that uses ferrite beads and a wave absorbing body constituted of a metal and resin or the like compound member. Also, Japanese Unexamined Patent Publication 127701/1992 discloses technology that uses a wave absorbing material for a part of a non-magnetic microstrip line. However, in either case, the wave absorber or the wave absorbing body merely plays an auxiliary role to suppress the high frequency component which cannot be absorbed.
In addition, in U.S. Pat. No. 4,301,428, a wire or cable that includes a conductive element with suitable electric resistance and a magnetic absorbing mixture is disclosed. The conductive element has a composite structure in which a non-conductive core constituted of a resin or glass fiber is covered with a thin conductive metal layer. The magnetic absorbing mixture is non-conductive and covers the conductive element. However, since imposing an electric resistance on the signal line causes attenuation in the signal component as well as removal of the noise component, it does not suit applications that handle, for instance, micro signals. In addition, this prior art technology only discloses a wire and makes no reference to a signal transmission element.
Connectors pose a similar problem to that discussed in regard to signal transmission elements. Generally, connectors have been developed with the emphasis on achieving signal transmission with low dissipation from low frequencies through the high frequencies. Known connectors which satisfy such needs include the SMA3.5 mm type, the SMA7 mm type, the BNC type and the N type. Each of these connectors is usually constituted with one or more signal lines, a ground line and is structured in such a manner that insulation is achieved between signal lines and the ground by an insulator constituted with a resin, such as acrylic or Teflon. The characteristic impedance of the connector are determined by its shape and the body constant of the resin. In many cases it is 50 .OMEGA..
As explained earlier, connectors have been developed with the emphasis on achieving signal transmission with low dissipation from the low frequencies through the high frequencies. However, with the recent advances in electronic technology, the advancement of digitalization of electronic circuits and the rapid development toward higher frequencies, the need for reducing high frequency noise in the GHz bands has increased drastically. In order to reduce the high frequency noise in the GHz bands, it is necessary to provide a connector with a low pass function that achieves removal of undesired high frequency components through absorption in the high frequency range. Yet, reflecting upon this background situation, it is obvious that it is extremely difficult to achieve a connector that suppresses the undesired signal components in the high frequency range of 1 GHz or higher through absorption.
The electronic part described above, which may be a signal transmission element or a connector, is intended to be used as a noise removal element only, a function that has no relation to the original function of the circuit structure. By adding these electronic parts to a circuit, frequency components in the high frequency range can be attenuated through absorption. By adding an attenuating factor that acts by absorption on frequency components in the high frequency range to a circuit board that is normally used when structuring an electronic circuit, in the form of an electronic part, such as a signal transmission element, a connector or the like, the onus on the rest of the circuit for attenuating such high frequency components through absorption can be reduced. There is even a possibility that electronic parts such as signal transmission elements and connectors that are used strictly as elements for noise removal, unrelated to achieving the original function, can be omitted.
Various attempts have already been made to suppress radiation of high frequency noise components to the outside of the circuit and, at the same time, to prevent the entry of high frequency noise components from the outside of the circuit, by shielding the circuit board itself. One example of this is the electromagnetic wave shielded printed circuit board described on page 155 of Electronic Parts Catalogue '92 compiled by the Parts Management Committee of the Japan Electronic Machinery Industry Association. However, in this case, it is necessary to enclose the board with a shielding layer, making the structure complex and, as a result, it is difficult to apply it in a limited way, to specific circuits formed on the circuit board. In addition, it is obvious when reflecting upon the background situation, that it is extremely difficult to achieve a circuit board that suppresses the undesired signal components in the high frequency range of 1 GHz or higher through absorption.